This application claims the priority of Japanese Patent Application No. 3-175596, filed Jul. 16, 1992, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hydraulic driving system equipped with a variable capacity type swash plate piston pump. More particularly, this invention is directed to a hydraulic driving system capable of discharging a fixed quantity of operation oil from the piston pump.
2. Description of the Related Art
Generally, an air conditioner mounted in a vehicle uses the vehicle's engine as power source, and is driven by a hydraulic driving system, which includes a hydraulic motor and a swash plate piston pump. Japanese Unexamined Utility Model Publication No. 62-97302 discloses a hydraulic driving system that drives an air conditioner. This system has its swash plate piston pump coupled drivably to the engine of the vehicle. This piston pump feeds a fixed quantity of operation oil to the hydraulic motor. A restriction is provided in an oil passage which connects the piston pump to the hydraulic motor. This system further comprises a flow control valve which controls the discharge quantity of the operation oil of the piston pump, based on the difference between the pressures at the input and output sides of the restriction.
The swash plate piston pump is provided with a spring for urging the swash plate in a direction so as to increase the tilt angle. The piston pump is also provided with tilt angle restriction means for restricting the increase in the tilt angle of the swash plate against the force of the spring. The spring and tilt angle restriction means constitute a mechanism for controlling the tilt angle of the swash plate. The tilt angle restriction means includes a control cylinder and a control piston. The control valve controls the oil pressure acting on the control cylinder, based on the difference in pressure between the input and output sides of the restriction. The oil pressure of the control cylinder acts on the swash plate via the control piston. The tilt angle of the swash plate is controlled on the basis of this oil pressure and the force of the spring. Consequently, the discharge quantity of the operation oil from the piston pump is adjusted in accordance with the tilt angle.
However, according to the conventional hydraulic driving system, when the engine stops or when the piston pump does not move, the tilt angle of the swash plate becomes greater, thus maximizing the discharge quantity. At the maximum tilt angle of the swash plate, the maximum load is applied to the engine, making it difficult to start the engine.
To start the engine more easily, the conventional hydraulic driving system has a clutch mechanism which is disposed intermediate the engine and the piston pump. The conventional hydraulic driving system deactivates the clutch mechanism, to functionally disengage the engine from the piston pump at the time the engine is started, and to activate the clutch mechanism to functionally re-engage them after the engine is started. In this manner, the load on the engine can be reduced to a minimal level.